1. Field of the Invention
This invention relates generally to borehole logging apparatus and methods for performing nuclear radiation based measurements. More particularly, this invention relates to a new and improved apparatus for effecting formation density logging in real time using gamma rays in a measurement-while-drilling (MWD) tool.
2. Background of the Art
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable also provides communication channels for sending information up to the surface. It thus becomes possible to measure some parameter of the earth""s formations as a function of depth, that is, while the sonde is being pulled uphole. Such xe2x80x9cwirelinexe2x80x9d measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
A wireline sonde usually transmits energy into the formation as well as a suitable receiver for detecting the same energy returning from the formation. These could include resistivity, acoustic, or nuclear measurements. The present invention is discussed with reference to a density measurement tool that emits nuclear energy, and more particularly gamma rays, but the method of the present invention is applicable to other types of logging instruments as well. Wireline gamma ray density probes are well known and comprise devices incorporating a gamma ray source and a gamma ray detector, shielded from each other to prevent counting of radiation emitted directly from the source. During operation of the probe, gamma rays (or photons) emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. In photoelectric absorption and pair production phenomena, the particular photons involved in the interacting are removed from the gamma ray beam.
In the Compton scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered toward the detector. Many of these never reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process of the pair production process. The scattered photons that reach the detector and interact with it are counted by the electronic equipment associated with the detector.
Examples of prior art wireline density devices are disclosed in U. S. Pat. Nos. 3,202,822, 3,321,625, 3,846,631, 3,858,037, 3,864,569 and 4, 628,202. Wireline formation evaluation tools such as the aforementioned gamma ray density tools have many drawbacks and disadvantages including loss of drilling time, the expense and delay involved in tripping the drillstring so as to enable the wireline to be lowered into the borehole and both the build up of a substantial mud cake and invasion of the formation by the drilling fluids during the time period between drilling and taking measurements. An improvement over these prior art techniques is the art of measurement-while-drilling (MWD) in which many of the characteristics of the formation are determined substantially contemporaneously with the drilling of the borehole.
Measurement-while-drilling logging either partly or totally eliminates the necessity of interrupting the drilling operation to remove the drillstring from the hole in order to make the necessary measurements by wireline techniques. In addition to the ability to log the characteristics of the formation through which the drill bit is passing, this information on a real time basis provides substantial safety advantages for the drilling operation.
One potential problem with MWD logging tools is that the measurements are typically made while the tool is rotating. Since the measurements are made shortly after the drillbit has drilled the borehole, washouts are less of a problem than in wireline logging. Nevertheless, there can be some variations in the spacing between the logging tool and the borehole wall (xe2x80x9cstandoffxe2x80x9d) with azimuth. Nuclear measurements are particularly degraded by large standoffs due to the scattering produced by borehole fluids between the tool and the formation.
U.S. Pat. No. 5,397,893 to Minette, the contents of which are fully incorporated herein be reference, teaches a method for analyzing data from a measurement-while-drilling (MWD) formation evaluation logging tool which compensates for rotation of the logging tool (along with the rest of the drillstring) during measurement periods. The density measurement is combined with the measurement from a borehole caliper, preferably an acoustic caliper. The acoustic caliper continuously measures the standoff as the tool is rotating around the borehole. If the caliper is aligned with the density source and detectors, this gives a determination of the standoff in front of the detectors at any given time. This information is used to separate the density data into a number of bins based on the amount of standoff. After a pre-set time interval, the density measurement can then be made. The first step in this process is for short space (SS) and long space (LS) densities to be calculated from the data in each bin. Then, these density measurements are combined in a manner that minimizes the total error in the density calculation. This correction is applied using the xe2x80x9cspine and ribsxe2x80x9d algorithm and graphs such as that shown in FIG. 1. In the figure, the abscissa 1 is the difference between the LS and SS densities while the ordinate 3 is the correction that is applied to the LS density to give a corrected density using the curve 5.
U.S. Pat. No. 5,513,528 to Holenka et al teaches a method and apparatus for measuring formation characteristics as a function of azimuth about the borehole. The measurement apparatus includes a logging while drilling tool which turns in the borehole while drilling. The down vector of the tool is derived first by determining an angle xcfx86 between a vector to the earth""s north magnetic pole, as referenced to the cross sectional plane of a measuring while drilling (MWD) tool and a gravity down vector as referenced in said plane. The logging while drilling (LWD) tool includes magnetometers and accelerometers placed orthogonally in a cross-sectional plane. Using the magnetometers and/or accelerometer measurements, the toolface angle can usually be determined. The angle xcfx86 is transmitted to the logging while drilling tool thereby allowing a continuous determination of the gravity down position in the logging while drilling tool. Quadrants, that is, angular distance segments, are measured from the down vector. Referring to FIG. 2, an assumption is made that the down vector defines a situation in which the standoff is at a minimum, allowing for a good spine and rib correction. A drawback of the Holenka method is that the assumption of minimum standoff is not necessarily satisfied, so that the down position may in fact correspond to a significant standoff: without a standoff correction and the results may be erroneous.
In a centralized or stabilized tool, the standoff will generally be uniform with azimuth. Holenka (U.S. Pat. No. 5,513,528) and Edwards (U.S. Pat. No. 6,307,199) also show how azimuthal measurements of density may be diagnostic of bed boundaries intersected by an inclined borehole. In the absence of standoff corrections, this can only be a qualitative measurement and the absolute density measurements may be suspect.
There is a need for a method of determining azimuthal density variations from measurements made by a MWD logging tool. Such a method preferably make corrections for the effects of standoff on the measurements. The present invention satisfies this need.
The present invention includes a logging-while-drilling method of determining azimuthal variations of density in a borehole. A logging tool is conveyed on a drill collar includes a long spaced (LS) and a short spaced (SS) nuclear sensor. Measurements are made using the nuclear sensors on the logging tool over a time interval while rotating the tool with the drill collar. Standoffs and azimuths corresponding to each of said LS and SS measurements are determined. A plurality of azimuthal sectors is defined around said logging tool. A plurality of standoff bins is defined within each sector. A processor is used for determining from the LS and SS measurements an azimuthal bulk density corresponding to each of said plurality of sectors that is corrected for the standoff of the sensors within that sector. A plurality of compensated sector data is used to generate a geosteering azimuthal compensated density.
In a preferred embodiment of the invention, the standoff measurements are made using an acoustic caliper while the tool azimuth is determined using a magnetometer in conjuriction with borehole survey information. A plurality of spatial bins is defined by azimuthal sector and standoff. Within each spatial bin, a compensated density is determined using the LS and SS measurements and the associated standoff. Compensated density determinations within an azimuthal sector are combined to give an azimuthal bulk density measurement. Relative strike and dip may be determined by analysis of the azimuthal density variation. The azimuthal density measurements may further be combined to give density differences between an xe2x80x9cupxe2x80x9d and a xe2x80x9cdownxe2x80x9d direction (and xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d). This difference may be used for controlling the drilling direction or as an indicator of proximity to a nearby interface.